It was not surprising that Supermarine asked Mitchell to design an entry for the seaplane competition, which was to commence on 1 September of that year. By later standards, the requirements for a successful entry were extremely modest. In the amphibian category, these included seating accommodation for a minimum of two passengers, a range of 350 nautical miles at 1,000ft and at a speed of not less than 70kt, and a load carrying capability of 500lb, to include passengers and life belts but not including crew. There was also a requirement for a flight of three minutes to check if the machine would fly itself unaided. Additionally, the entries were required to clear balloons at a height of 25ft from a 400-yard take-off run.
From the experimental station at Felixstowe, the amphibian competitors were required to take off, pass as high as possible between marker boats 600 yards from the start buoy and land at the experimental (land) station at Martlesham Heath. Taxiing on water had to include figures of eight, taking off and landing in rough weather, and mooring out for at least twenty-four hours in moderate weather. These marine trials were not unlike those that Mitchell’s Schneider Trophy racers had to complete before the actual flying contests, and of course reflected the same concern to develop aircraft that had practical seagoing features.
The Commercial Amphibian
As the Commercial Amphibian was the first comprehensive design by Mitchell, it is surely very understandable that the end product would be a conservative one. Even if Mitchell had been an experienced aircraft engineer at this time, he would still, in all probability, have followed previous best practice, in view of the little theoretical data that was available and as wind tunnel experimentation or tank testing (for flying boat hulls) was not available to his small firm.
Also, there were only about twenty weeks separating the announcement of the competition and the trials, leaving little time for innovative thinking. Flight reported that ‘for the Martlesham amphibian trials the Supermarine Company designed and completed a flying boat in all respects in ten weeks from the time when the first drawing was commenced to the time the aircraft was in the air, the actual building time being four weeks’. Not surprisingly, therefore, Supermarine described the new design as ‘practically a “Channel” type boat, with a wheeled undercarriage hinged on each side’ although, in fact, side views show considerable changes in the Mitchell design (see opposite).
Channel
Commercial Amphibian.
Based on a Channel airframe, the Commercial Amphibian had a biplane layout in which similar dimensions of height and length were adopted, and the sea rudder was similarly placed – vertically below the leading edge of the tailplane – but now converted to act also as a skid when taxiing over land. The wing-tip floats were also of the Channel sort and the oval hull and the general arrangement of its built-on planing surfaces employed the Linton Hope/Channel principles of hull construction.
On the other hand, Mitchell also incorporated features of a much smaller aircraft, the Sea Lion I. The fin and rudder outlines were similar, although a proportional increase in surface area above the tailplane allowed the designer to provide a more symmetrical appearance to the fin. And the Sea Lion’s outwardly raked inter-plane struts were repeated in the new, and larger, machine. (Thereafter, it would seem that Mitchell preferred the simplicity of equal span wings supported at right angles by the inter-plane struts.)
Between the Amphibian’s struts there were canvas stabilising screens, full length between the inner pairs and quarter length between the middle ones. These screens were relatively uncommon by this time, but survived on several later Supermarine designs as well as on the Channel and Sea Lion, perhaps mainly to protect the engine and propeller from spray on take-off or landing; but it was the present machine that was most extensively fitted with them and, in this respect, it did not look particularly like an advanced design.
As many of the features from the Channel and the Sea Lion I were, thereafter, abandoned by Mitchell, the present design can be regarded as something of a ‘time capsule’, a summing up of earlier practices rather than a statement of the way forward. But Mitchell also showed an early instance of boldness by abandoning the biplane tailplane and twin rudders of the Channels (still evident in the Handley Page HP 42 of the 1930s) in favour of a single fin and tailplane.
It is also worth noting that the competition rival, Vickers Viking III, went through three more variants before the Mark VII, the Vanellus, appeared five years later with a more modern-looking single tailplane. Also, Mitchell’s rudder was a departure from the minimalist approach of previous Admiralty-inspired rudders – perhaps his work alongside Hargreaves on the Sea Lion I had had some influence in this respect. Additionally, Mitchell significantly remodelled the nose with a prominent boat-like entry to counter spray, a feature which was to prove successful in his future Sea Eagle, Scarab and Seagull designs.
The Commercial Amphibian with ground handlers, at Martlesham. (Courtesy of E.B. Morgan)
A further novel feature was Mitchell’s design for a retracting undercarriage, necessitated by the Air Ministry competition being for amphibian aircraft only. At this time an American land plane, the Dayton-Wright RB-1 Racer, had an innovative fully retracting landing gear, designed especially for the Gordon Bennett race of 1920, whereas the present concern was merely to lift the wheels out of the water, in order to facilitate take-off and alighting.
The first European design of this sort was the Sopwith Bat Boat of 1913, which, like the present rival, Viking, had a mechanism that rotated the wheels upwards and forward. Supermarine’s concern for ‘boats which fly’ offered no previous experience of retractable undercarriages for Mitchell to call upon, and so it is noteworthy that for his specially designed mechanism he chose a geometry that displaced the wheels outwards rather than forward – thus avoiding any change of trim when the wheels were moved up or down. (Mitchell retained this sideways mode of retraction for all of his future amphibian undercarriages.)
One other particular feature of the Commercial Amphibian must also be mentioned: the enclosed passenger cabin. The competition’s intention of ascertaining ‘the best type of Float Seaplanes or Boat Seaplanes which will be safe, comfortable and economical’ might have seemed to make an enclosure for passengers inevitable, but it should be noted that the other two amphibian entries had open cockpits for their three passengers, one seated next to the pilot and the other two side-by-side behind. Open cockpits at this time were the norm, and they saved weight, but they were far from ideal in northern climates – one remembers Cozens’ previous description of Channel passengers looking ‘wet and miserable as they got into a boat that was rowed out to meet them’.
Supermarine’s concern that passengers should not be in the open might also have resulted from the experience of Supermarine’s pilot, Henri Biard, on 30 September 1919, on the Channel Flying Boat Service to Le Havre. He recorded that the weather that day had deteriorated into a gale with sleet and snow but, with a flask of rum supplied by Scott-Paine for heating, a Belgian financier braved the open front cockpit of the aircraft. The cold was such that the passenger, a Captain Alfred Loewenstein, tried to pass the flask to the rear cockpit but only succeeded in causing the spirit to blow back into the pilot’s eyes. Thereafter, the passenger tried to raise an umbrella, obviously unaware that the aircraft slipstream might blow it into the propeller – which was directly behind the two men. As it was impossible to converse with the passenger in the front cockpit, Biard had to resort to hitting Loewenstein about the head, whereupon he disappeared into the well of the cockpit. (The Belgian was to become a ‘mystery of flying’, on 4 July 1928, when he disappeared from a Fokker FVII over the Channel.)
This glimpse into the pioneering days of aviation might seem amusing (though not to the pilot at the time), as was the arrival of the Supermarine crew for the Air Ministry competition dressed in heavy jerseys and sea boots. The Vickers people turned up in sailor hats with ‘Viking III’ in gold on their hat-bands and the Aeroplane at the time not
ed that ‘all the competitors treat the affair as a very good joke’.
Despite the apparently light-hearted or amateur approach to the event, the same correspondent did also note that the amphibian entrants hedged their bets by reserving their maritime tests until last ‘as they wanted to complete land tests before chancing damage to their machines by awkwardly handled launches or a sudden squall’.
No significant adjustments or replacements to the Mitchell aircraft were required, despite its one-off design and the short notice of the competition. Supermarine publicity made the most of it, saying that the aircraft ‘put up an extraordinarily good show in that competition. It completed all the tests satisfactorily, and was only beaten by competitors with engines of considerably greater power in the matter of speed and climb’. The judges also noticed with approval an effective tiller arrangement for steering while taxiing on water, the other equipment for sea use, and the way in which the shape of the forward part of the hull kept spray off the passengers’ compartment.
It might be expected that the company’s marine experience would produce such comments. Equally, it might not be too surprising that Mitchell’s novel undercarriage gave rise to criticism for being none too clean, from the mechanical and maintenance points of view. The lateral control of the Commercial Amphibian was also considered not immediately responsive enough and the wing-tip floats too small and inadequately secured. On the other hand, a comparison between the slab-sided Consuta plywood sheet approach of the Vickers entry and the boat-like hull that Mitchell inherited, showed that he had had the good fortune to have joined a firm with a technique of hull building that was to stand the firm and its chief designer in good stead for the rest of that decade.
An increase of nearly 150sq.ft of wing area compared with the Channel had been estimated by Mitchell as being needed to address the performance specifications of the competition and to lift the additional weight of the amphibian landing gear. A more powerful but heavier engine, the Rolls-Royce Eagle VIII, was also used. Unfortunately, the result was a loss of certain competition points when, otherwise, it would have taken first prize.
Despite its passenger-carrying capacity of only two – the minimum allowed by the competition rules – the final report on 11 October stated that ‘the results achieved for amphibians show that considerable advance has been attained … and the competing firms deserve congratulations on their enterprises’ and the second prize money of £4,000 was doubled as ‘the proportion of the monetary awards does not adequately represent the relative merits of the first two machines’. (A Fairey floatplane with an added wheel attachment came third and was awarded £2,000.)
One can imagine how the new Mrs Mitchell must have felt at this promising start to her husband’s career but, from a technical point of view, it was a modest beginning, to be sure. Nevertheless, as we shall see later, many of its features and its overall performance gave rise to a call from the Air Ministry for a further development of this machine, which led to the Sea Eagles and the Seagulls between 1923 and 1926.
THE SEA EAGLES
The 1920 commercial aircraft competition had been a reflection of the inclusion in the Government’s Air Estimates of an allocation for civil aviation (although it represented something less than 2.5 per cent of the total), and in June 1922 the Air Ministry gave approval for an air service between Southampton, Cherbourg and Le Havre. The route, with a subsequent extension to the Channel Islands, was to be operated by an air service named the British Marine Air Navigation Company, and Hubert Scott-Paine and James Bird of Supermarine were to be its directors. Not surprisingly, the first Supermarine aircraft for this service was already being built when the Air Ministry granted the company a subsidy of £10,000 and agreed to pay £12,000 for aircraft and spares (later revised substantially downwards as the air miles generated were less than the company had expected to fly).
A Sea Eagle with Biard aboard. Note the sea anchor below the cockpit and the original, single fuel tank on the top wing. (Courtesy of E.B. Morgan)
The aircraft designed for this service was named the ‘Sea Eagle’ as it was to be powered by the Rolls-Royce Eagle IX engine. For this machine Mitchell continued the customary pusher configuration for single-engined flying boats and went back to the more boat-like hull shape of the larger Channel and Commercial Amphibian designs. In fact, the fore section of the Sea Eagle, with its high, pointed prow, enclosed accommodation for passengers, large windows and grab-rails on the roof, resembled a cabin cruiser of the time. As the two planing steps were also joined by a continuous hard chine which ran three-quarters of the hull length, it embodied, more than any other Mitchell design, the original Pemberton-Billing concept of ‘boats which fly’.
Wing folding was again adopted and a forward-folding arrangement was again employed – which reduced the width of the Sea Eagle by 54 per cent, although increasing the length by 15 per cent. This configuration had the structural advantage of siting the folding mechanism at the main spar with no possibility of a wing folding backwards in flight. This arrangement, however, necessitated a cut-out in the leading edge of the wings, which did nothing for aerodynamic efficiency.
Mitchell adopted the practice of gravity feed for the engine of this latest flying boat with apparently few qualms about stability problems, for the fuel tank (and subsequently a second tank) was attached to the top of the wing centre-section. On 28 June, a Flight correspondent wrote that that ‘this machine represents a great step forward in the development of the seaworthy amphibian’ having appreciated a ‘most important innovation’ that, in place of the usual tank in the hull, ‘the main petrol tank has been mounted on top of the top plane, so that direct gravity feed, with its attendant simplicity and freedom from breakdown, can be used’.
The writer also added:
The fact that the engine is mounted high above and some distance aft of the cabin has resulted in reducing the noise audible in the cabin to a minimum, and as a matter of fact, in the ‘Sea Eagle’ it is possible for the passengers to converse in ordinary tone of voice, without having to shout to one another. [One remembers Biard’s earlier ‘communication problem’ with Loewenstein in the Channel flying boat.]
One departure from all previous (and future) biplane practice was the use of a pronounced stagger of the two wings, as the weight of the forward passenger cabin and its six passengers necessitated bringing the centre of lift of the top wing well forward of the engine. One notices that Mitchell’s usual preference was for the simplicity of directly opposed wings.
The first completed Sea Eagle made its maiden flight in June 1923, and received its Certificate of Airworthiness on 11 July. Two days later, Supermarine entered the new aircraft in the King’s Cup Air Race that had been initiated the year before, also to encourage aviation development. As it was a handicapped event, the entry of a commercial flying boat might not seem too strange, but the carrying of four passengers must have had much to do with the company being mindful of publicity generated by air races. Unfortunately, circumstances involving a burst tyre and its replacement led to the aircraft being disqualified.
On the 5th of the next month, the Director of Civil Aviation at the Air Ministry, Sir Sefton Brancker, came to Southampton and was given a display of the machine’s ability to negotiate the (usually) crowded seaway, as well as a demonstration flight. He announced himself to be well satisfied with the Sea Eagle’s potential contribution to the development of civil aviation, both in terms of performance and comfort and, along with other senior members of his department, had another flight nine days later.
Particular comment was made on the very sensible placing of the passengers, and in the following publicity the company makes reference to the advantages of this arrangement:
This machine was specially designed as a commercial amphibian or flying boat for passenger carrying work. It carries six passengers and pilot, with fuel for a distance of 230 miles. Extra tankage is fitted so that the range can be increased by reducing the number of passenger
s.
The passengers are accommodated in a roomy cabin in the fore part of the hull. This cabin is very comfortably fitted out. Its position in front of the engine makes it very quiet and free from engine exhaust, gases, oil, etc.
It is very efficiently heated and ventilated, and is fitted with sliding triplex windows along the two sides for use in the warm weather. The machine is very strongly built and very seaworthy, and has proved itself quite safe in the roughest of seas usually experienced in the Channel. It is fitted with either a Rolls-Royce ‘Eagle IX’ engine of 360hp or a Napier ‘Lion’ of 450hp.
Supermarine’s experiences with the Channel service and with the Commercial Amphibian had obviously influenced Mitchell to give considerable thought to the enclosed cabin arrangement, such that one passenger recorded descending into the Sea Eagle and finding ‘a delightful little room’ that the company had fitted with ‘reposeful armchairs’.
Intermittent proving services began in August, and regular daily services between Southampton and Guernsey began on 25 September 1923. As such, it constituted the very first British scheduled flying boat service and was advertised to leave Woolston at 11.15 a.m. and return from St Peter Port at 3.30 p.m. (the French section of the service did not materialise). The service, often with breaks due to bad weather, continued with the Sea Eagles for the next five years, even though the single fare to the Channel Isles was not cheap for the 1920s at over £3 single and £7 return. Compared with boat transport, however, the normal flight time of one and a half hours was very attractive, although in adverse wind conditions it might be almost an hour more.
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